The invention relates to a filter cloth intended for use in a pressure filter based on diaphragm extrusion which comprises at least two filter chambers and in which the filter cloth is guided through the filter chambers arranged one after the other in the direction of movement of the cloth so that in the filter chambers, the opposite sides of the filter cloth are alternately against the slurry to be filtered which contains liquid and solids, the filter cloth being symmetrical in respect of the filtering ability, and the solids separated from the slurry in said filter are conveyed out of the filter chamber by means of the filter cloth.
Removal of liquid, i.e. solid/liquid separation, is needed in the mining industry, metal processing, chemical industry and in preparation processes of foodstuffs and pharmaceuticals, for instance. For this purpose there are various filtering apparatuses the operating principles and properties of which differ from one another. One known filter type used in the treatment of slurry consisting of liquid and solids is a vertical chamber filter which has a variable volume and utilizes diaphragm extrusion. A pressure filter known by the product name Larox(copyright) also works on this principle. The operation and structure of this filter will be described below in FIGS. 1 to 3f. The filter comprises several superimposed horizontal filter plates which define filter chambers. A filter cloth is arranged to travel through the openable and closable filter plates of each chamber. The slurry to be filtered is fed into the chambers to the other side of the filter cloth, after which it is pressed against the filter cloth by means of an expanding elastic diaphragm. In that case the liquid in the slurry is squeezed through the cloth and the solids remain on the other side of the cloth, forming a xe2x80x98cakexe2x80x99. After compression and the other phases the pack formed by the filter plates is opened and the filter cloth is moved with respect to the filter chambers to remove the solids from the chambers. The filter cloth travels, guided by suitable turning rolls or the like, from one of the superimposed chambers to another, and thus the slurry is alternately placed on the different sides of the filter cloth in the filter chambers arranged one after the other in the direction of movement of the cloth. Consequently, the filter cloth needs to be symmetrical in respect of its filtering ability to provide filtrate of equal quality from each filter chamber.
The requirements set for the properties of the filter cloths intended for the above-mentioned use are high. Not only do they function as the filtering medium, but also as a conveyor belt when conveying the solids cake during the discharge phase of the filtration process out of the filter chambers. In the largest filters the solids separated during one phase and conveyed by the filter cloth may weigh even 20 000 kilos. Particularly in the discharge phase the filter cloth is thus subjected to a high tensile stress. Furthermore, during use scrapers and washing devices, for example, subject the filter cloth to considerable wear. The conditions in which the cloths must operate are also demanding, i.e. in addition to mechanical stress the cloths are subjected to high temperatures, temperature changes, high pressure and varying pH conditions, for example. The filter cloths need to be replaced from time to time not only because of wear and tear, but also because of a decreased filtering ability resulting from clogging and dirtying. As regards the use of the filtering apparatuses, it would be advantageous if the useful life of the filter cloth could be increased because replacement of the cloth causes considerable costs and production breakdowns.
The object of the present invention is to provide a novel filter cloth which eliminates problems associated with the prior art solutions.
The filter cloth of the invention is characterized in that it comprises a middle layer both surfaces of which are provided with protective layers which form the outer surfaces of the filter clothr.
The basic idea of the invention is that the filter cloth intended for use in a pressure filter based on diaphragm extrusion comprises at least three layers attached to one another. In that case both outer surfaces of the filter cloth are provided with separate protective layers. The protective layers are formed so that the filter cloth is at least functionally symmetrical, i.e. its fitering ability and other operating characteristics are substantially similar on both sides of the cloth. The basic idea of a preferred embodiment of the invention is that the protective layers are made to resist wear by using wear-resistant fibres or threads and bond structures between them. Thus the protective layers function as wear surfaces which effectively protect the middle layer of the filter cloth. The basic idea of another preferred embodiment of the invention is that the protective layers are denser than the middle layer, thus functioning as the actual filtering layers of the cloth.
An advantage of the invention is that the useful life of the filter cloth can be increased compared to the prior art cloths. Thanks to the longer useful life, replacement of the filter cloth will cause fewer breaks in production and the other costs related to the replacement will also be lower. A further advantage is that in addition to durability and density, the other filtering and operating characteristics of the cloth can be influenced by the selection and dimensioning of the protective layers, if necessary. According to an embodiment of the invention, the quality of the resulting filtrates can be improved by means of a filter cloth which is denser than the cloths used at the moment. The dense filter cloth is also easier to wash and clean since the dirt is not absorbed into the cloth as easily as before. Thanks to the multilayer structure, the middle layer of the filter cloth can be made looser than before, and thus the required density of the cloth is provided by means of the protective layers.